1. Field of the Invention
The present invention relates to a machine tool having a rotary table.
2. Description of the Related Art
A machine tool having a rotary table is equipped with a clamping mechanism for determining the phase of the rotary table and maintaining that phase. As described in Japanese Patent Application Laid-Open No. 2009-83013 for example, such a clamping mechanism maintains the determined phase by pushing a pressing member or the like against a disk integral with the rotary table (clamping), or conversely, permits rotation of the rotary table by releasing the pressing member from the disk (unclamping).
Japanese Patent Application Laid-Open No. 2009-248242 describes a clamping device incorporated in a rotary table that checks clamping after a clamping operation by determining the state of clamping based on positional deviation information.
Traditional clamping mechanisms confirm that clamping and unclamping operations are completed by means of a sensor before performing the next operation. Referring to the timing diagram shown in FIG. 4, a procedure of operations (clamping (unclamping)) associated with such a clamping mechanism will be described.    (1) A command signal to start clamping (or unclamping) is output by a numerical controller of the machine tool.    (2) Mechanistic clamping (or unclamping) operation of the clamping mechanism is started.    (3) Completion of the mechanistic clamping (or unclamping) operation of the clamping mechanism is detected by a sensor.    (4) A command signal for the next operation of the machine tool is output by the numerical controller.
As shown in FIG. 4, the time (clamping mechanism operation time) from output of the clamping (or unclamping) start command signal in (1) to output of the command signal for the next operation of the machine tool by the numerical controller in (4) is Ta. Because the clamping mechanism operation time Ta is not constant, a current clamping mechanism determines Ta by measurement with a sensor and the numerical controller of the machine tool. The actual operation time taken for clamping (or unclamping) (actual clamping mechanism operation time) is time Tb from output of the clamping (or unclamping) start command signal in (1) to start of clamping (unclamping) operation of the clamping mechanism in (2).
It can be thus deemed that a clamping (unclamping) process involves an extra processing time Tc equal to clamping mechanism operation time Ta minus actual clamping mechanism operation time Tb (i.e., Tc=Ta−Tb). The processing time Tc is substantially constant as it essentially depends on the throughput of the tool (time required for signal transmission and time required for signal processing). For a fast clamping mechanism that performs clamping (unclamping) at high speed, the processing time Tc accounts for a relatively large portion of clamping mechanism operation time Ta, being a non-negligible length of time.